In conventional direct injection type internal combustion engines, for realizing a stratified charge, a ball-like depression or a horseshoe-shaped enclosure generally is formed at the crown of a piston to trap fuel.
An example of a combustion chamber structure in the above conventional type of an internal combustion engine is disclosed in Japanese Patent Laid-Open No. 7-166872. In a four-valve type internal combustion engine disclosed in this publication, two intake ports each provided with an intake valve and two exhaust ports each provided with an exhaust valve are formed in positions substantially opposed to each other for a combustion chamber with a spark plug provided nearly centrally. Both intake ports are constituted as tumbling ports for imparting vertical tumbling flows to intake air which is introduced into a cylinder from the intake ports. Further, a slot is formed in a portion of the crown of a piston adapted to reciprocate within the cylinder which portion is positioned under the spark plug. The slot is formed so as to extend from a lower position between both exhaust valves toward a lower position between both intake valves, with one or more protuberances being formed on both right and left sides of the slot of the piston crown so as to extend from the exhaust valve side toward the intake valve side in an inwardly inclined manner in plan, to make the air-fuel ratio of introduced air-fuel mixture lean, thereby attaining low fuel consumption and low pollution.
Another example of a combustion chamber structure in such conventional type of an internal combustion engine is disclosed in Japanese Patent Laid-Open No. 8-246877. In the combustion chamber structure disclosed therein, a combustion chamber is defined between a recess formed in the underside of a cylinder head and a piston. A spark plug is disposed nearly centrally of a cylinder and one or plural intake valves are disposed at a position offset to one side of the cylinder. An intake port or ports are formed so that a tumbling flow is formed within the combustion chamber by an intake flow which enters the combustion chamber through the intake valve or valves. Further, a recess is formed radially in the piston crown so as to be narrower on the intake valve side and wider on the opposite side, thereby realizing a stratified charge of a rich air-fuel mixture in the vicinity of the spark plug positioned centrally of the cylinder.
A further conventional example is disclosed in Japanese Patent Laid-Open No. 9-105330. In a four-valve type internal combustion engine disclosed in this publication, two intake ports each provided with an intake valve and two exhaust ports each provided with an exhaust valve are disposed at positions substantially opposed to each other for a combustion chamber with a spark plug provided nearly centrally. Both intake ports are constituted as tumbling ports for imparting vertical tumbling flows to intake air which is introduced into a cylinder from the intake ports. Further, a first recess is formed in the crown of a piston in the cylinder at a position approximately just under both exhaust valves, the first recess being formed wide so as to serve also as a valve recess for both exhaust valves, while a second recess narrower and shallower than the first recess is formed in the crown of the piston at a position between and approximately just under both intake valves, the second recess being formed so as to communicate with the first recess, to prevent deceleration of both tumbling flows and enhancing the influence of both tumbling flows on the vicinity of the spark plug, thereby making the air-fuel ratio leaner.
A still further conventional example is disclosed in Japanese Patent Laid-Open No. 10-8968. A piston for an internal combustion engine disclosed in this publication includes two ridge portions each having an edge line extending in parallel with a crank axis. The ridge portions are formed on the piston crown in a sandwiching relation to a central part of the crown, and a recess constituted by a cylindrical surface having an axis parallel to the crank axis is formed between the ridge portions.
A still further conventional example is disclosed in Japanese Utility Model Publication No. 3-6827. According to this publication, a combustion chamber structure in an engine is formed in a pent roof shape whose section in a direction orthogonal to the direction of a cylinder row is generally triangular, while on a piston crown located at a lower position of the combustion chamber, and constituting part of the same chamber, there are formed squish zones at both side positions in conformity with the pent roof shape. The squish zones extend in the cylinder row direction, and with a recess formed between the squish zones. The recess extends up to piston ends in the cylinder row direction and is formed in an elliptic shape which is curved so that the outer periphery of the recess expands toward the exterior of the piston. The bottom of the recess has a spherical shape which gradually deepens toward the inner periphery from the outer periphery of the ellipse. Further, a spark plug is disposed nearly centrally of the recess to decrease the amount of hydrocarbon discharged.
A still further conventional example is disclosed in Japanese Utility Model Laid-Open No. 58-57528. A combustion chamber in a direct injection type internal combustion engine is disclosed in this publication. An edge portion of a piston cavity is projected toward a cylinder head to form a weir portion for minimizing the gap between it and the cylinder head when the piston reaches its top dead center.
A still further conventional example is disclosed in Japanese Utility Model Laid-Open No. 59-22958. A piston in an internal combustion engine, disclosed in this publication, has a ring-like protuberance and is formed on a piston crown.
A still further conventional example is disclosed in Japanese Utility Model Laid-Open No. 2-56816. According to a combustion chamber structure in an internal combustion engine disclosed in this publication, a spark plug is disposed nearly centrally of an upper wall of a cylinder head, the upper wall defining a combustion chamber and being formed in a pent roof shape. Three intake valves are disposed on one side of the upper wall. Of the three intake valves, the one located at the center is larger in diameter than the other intake valves located on both sides, and in a piston crown portion to which the central intake port faces, there is formed a curved recess extending in the axial direction of the same port.
A still further conventional example is disclosed in Japanese Utility Model Laid-Open No. 5-21132. According to a combustion chamber in an internal combustion engine disclosed in this publication, there is a cylinder head wherein intake valves and exhaust valves are arranged in a generally symmetric shape. A recess having a deep portion on the exhaust valve side is formed in a piston crown to increase the amount of an air-fuel mixture which reverse tumbles under the exhaust valves, thereby improving the state of combustion.
A still further conventional example is disclosed in Japanese Utility Model Laid-Open No. 6-12724. According to a combustion chamber structure in an internal combustion engine disclosed in this publication, a depression-like cavity is formed in a piston crown and two spark plugs are disposed in a cylinder head in such a manner that the respective tips face the interior of the cavity in a radially spaced relation to each other on one side in the cylinder. The cavity is formed in a sectorial shape in plan so as to be narrow toward the center of the piston in an area spaced radially away from the two spark plugs and wider toward the peripheral edge portion of the piston in an opposite area. This shape of the cavity is effective in increasing the burning speed in the area where the flame propagation distance is long.
In the conventional internal combustion engines, as shown in FIGS. 54 and 55, two layers of tumbling flows are created within a combustion chamber 218 of an internal combustion engine 202 when intake air is fed into the combustion chamber 218 form intake ports 214-1 and 214-2 through two intake valves 222-1 and 222-2. In this case, if a crown 208a of a piston 208 is formed flat as in FIGS. 54 and 55, there arises an inconvenience in that trapping of fuel is not carried out effectively.
In an effort to eliminate such inconvenience it has been proposed to form a ball-like depression or a horseshoe-shaped enclosure at the piston crown to trap fuel by the depression or the enclosure and realize a stratified charge.
In an internal combustion engine having a large bore diameter, a ball-like depression or a horseshoe-shaped enclosure is formed so as to effect trapping of fuel relatively easily. But in an internal combustion engine having a small bore diameter, say, 80 mm or less, it is difficult to retain an air layer around an air-fuel mixture present at a central part and thus difficult to realize a stratified charge. It has been desired to remedy this point.
Additionally, in the case where a ball-like depression or a horseshoe-shaped enclosure is formed at the piston crown of the internal combustion engine, the weight balance of the piston is lost and causes a mechanical loss such as an increase of piston side thrust.